X-rays can be used to make a detailed nondestructive examination of the internal structure of a material. For example, observation by transmission X-ray imaging has extensively been applied to such as the diagnosis of a living body in medical and pharmaceutical fields, the evaluation of various industrial products and an apparatus for the inspection of pieces of luggage in an airport. Other than those utilizing synchrotron radiation, the X-ray sources currently used include an X-ray tube based on the method of striking accelerated electrons on a metallic target in vacuum to emit X-rays from the target. As the X-ray source that can be obtained by this method has a size of several μm generally, the resolution obtainable by the X-ray radiography has remained in several μm, too, which has not been satisfactory.
It was reported by the present inventors that in around 1973, an X-ray photograph was taken of an ant with a microfocus X-ray source of 3 μm diameter to observe its body hair and blood vessels (see: Jinpei Harada and Masaru Kuribayashi, “High-Definition Images by Micro-focus X-ray Source”, Japan Society of Photography, vol. 65, No. 7, pp. 495-500, 2002). In this photographing, the X-ray target used was made of gold (Au) and had a thickness of 2 μm. While the X-ray radiographic imaging contrast has been said to be due to the scattering and absorption of X-rays, it is pointed out that the use of a microfocus X-ray source allows a phase-contrast image to be included and therefore to capture the contours of an object if it is such as a hair which is low in density and very small in thickness (see: S. W. Wilkins and four others, “Phase-contrast imaging using polychromatic hard X-ray”, Nature, vol. 384, pp. 335-338, 1996).
FIG. 12 is a diagram schematically illustrating a conventional X-ray source. In the Figure, applying an electron beam 100 to a target thin film 101 of thickness t is shown to generate X-rays 102. In this case, electrons 103 diffuse into the target thin film 101 with a diffusion length L.
X-rays are also used in the inspection of a semiconductor device, especially an ultra LSI (large Scale Integrated Circuit) by elemental analysis or X-ray photoelectron spectroscopy (XPS) with characteristic X-rays or fluorescent X-rays. For example, the Japanese laid open patent application JP H05-45306 A discloses a method for X-ray analysis in which X-rays radiated from a rotating target as the X-ray source are passed through a fine glass tube to generate a fine X-ray beam of 5 μm in diameter on a specimen. In this method, numbers of fine glass tubes are used to enhance the intensity of X-rays. For this reason, different focus sizes are apt to be produced from different fine glass tubes, and the arrangement is unsuitable for X-ray radiographic or transmission imaging. Further, there are also limits in machining such a fine glass tube. However, the requirement indeed exists for an X-ray source of still finer focus size to meet with semiconductor elements having a minimum feature size of 90 nm or 60 nm.
However, while reducing the focus size of a conventional X-ray source as shown in FIG. 12 requires reducing the spot size of an electron beam and increasing the density of electrons in the electron beam for irradiation, the problem arises that if this is done, electrons forming the beam still then diffuse within the X-ray target and fail to give rise to a fine focus X-ray source. Also, if the density of electrons is raised in the beam for irradiation, the problem emerges that this will cause the X-ray target material to be heated to a molten state and then to a sublimed or evaporated state, and will damage the target thin film 101.